This invention relates to a wave energy converter (WEC) designed to provide improved efficiency under normal operating conditions and to have improved survivability to large amplitude waves.
A WEC, as shown in FIGS. 1A and 1B, may include a float 100 and a shaft/spar 20 with a power take off device (PTO), 30, connected between the float and shaft. The float is generally designed to move in synchronism with the waves. The shaft 20 may be designed to be stationary (e.g., anchored to the sea bed as shown in FIG. 1A) or it may be designed so that it can move up and down, in phase with the float but with a time delay relative to the float and/or generally out of phase with the waves and the float, as shown in FIG. 1B, in a configuration which may be referred to as a “dual absorber”. In any case, the PTO is connected between the shaft and the float for converting their relative motion into useful energy (e.g., electrical power or different forms of mechanical energy).
The floats 100 of prior art WECs tend to be formed to be generally symmetrical (e.g., circular or square) about the x-y axes, as shown in FIG. 1C. The WECs used may be of the “point absorber” type where the term “point absorber” is generally defined to mean that the characteristic dimension of the float of the WEC is small in relation to the (longer) wave length of the waves, driving the WEC.
In many situations the amount of power that can be produced by a WEC is a function of the surface area of the float subject to be acted upon (lifted or lowered) by the waves. The buoyant force on the float can be estimated as the change in displaced volume of the float as a wave passes by. For waves having a very long wavelength impinging on a float (e.g., the wavelengths are much longer than the dimensions of the float in width or length), the change in displaced height of the float is essentially the same all over the surface of the float. For this case, the shape of the float is not significant in considering its power producing capability. However, for waves impinging on a symmetrical (e.g. circular) float having a wavelength comparable to the dimension of the float, when one side of the float is under the crest of the wave, the other side or edge of the float is not under the crest. When this occurs there is a cancellation effect. The buoyant forces of the wave do not act (e.g., lift) across the full surface area of the float. In this instance, the amount of power that can be produced is significantly reduced.
This may be better explained with reference to FIG. 1D which illustrates the effect of a wave on a symmetrical float (section B of 1D) and an asymmetrical float (section C of 1D). Section A of FIG. 1D shows a wave 901, having a period of 7 seconds, a wave height of 2 meters and a wavelength of approximately 75 meters. For purpose of illustration, waveform 901 is shown to have a peak value (crest) at point K, a lower value at a point L, which is 5.5 meters away from the crest, and a still lower value at a point M, which is 11 meters away from the crest. Consider now a prior art circular float 100 (as shown in section B of 1D) having an outer diameter of 11 meters which is subjected to waveform 601. As shown in the drawing, the left side of the float (K) lines up with the peak of the wave crest. It is evident that, for this wave condition, only part of the float's surface area will be subjected to the full force corresponding to the wave amplitude. The rest of the float will be subjected to a lower force and may even be pushing down, canceling the up-lifting force. Thus, the power developing/producing capability of the float 100 is significantly reduced. For waves whose wavelength is even less than that shown for wave 901, it is evident that even less power can be developed and produced.
To overcome this problem, it is proposed that the float be made asymmetrical, as per the top view shown in section C of FIG. 1D. For example, there is shown an elliptical float 10 with a length of 22 meters (long side) and a width of 5.5 meters (short side). The area of the symmetrical float in B of FIG. 1D is essentially the same as the area of the asymmetrical float in C of FIG. 1D. As may be seen, essentially the full surface area of the asymmetrical float will be subjected to the full force of the wave 901. So, from the point of view of power production it is desirable to have an asymmetrical float with its longer side facing the direction from which waves are incident. Clearly, the non-symmetric float has preferred characteristics for wave energy conversion for waves having shorter wave lengths, relative to the size of the float. That is, for waves having shorter wave lengths, relative to the size of the float, a properly oriented non-symmetrical float of similar area to a symmetrical float will convert wave energy to a useful form of electricity more efficiently, i.e., more of the power in the wave will be converted to a useful form of power than for a prior-art symmetrical float.
Therefore, for waves whose wavelengths are within a “normal” range (e.g., ranging from less than a 5 second period to more than a 14 second period), it is desirable to have an asymmetrical float to capture more wave energy and optimize wave power conversion. However, Applicants recognized that a significant drawback exists to the use of the asymmetrical float because: (1) the direction of the incoming waves may vary undoing the benefits sought; and (2) it has greater susceptibility to being damaged under storm conditions. That is, where the typical wave amplitude is less than 4 meters, the WEC is designed to be operational for and survive the typical wave condition. However, under storm conditions where the wave amplitudes are greater than normally expected (e.g., the waves have amplitudes greater than 4 meters) greater buoyant forces are applied to the asymmetrical float and significantly higher forces are developed between the float and spar tending to damage the WEC and its PTO. In consideration of these problems, there is no known WEC system with an asymmetrical float which is raised and lowered by the waves.
Thus, while it is desirable to have the long side of an asymmetrical float facing incoming waves for improved wave energy conversion, there is a problem with the survivability and operability of the WEC under storm and varying wave conditions.